This invention relates generally to bandgap voltage reference circuits, and in particular, to bandgap voltage reference circuits and related methods that add two currents having respectively opposite polarity temperature coefficients to generate a substantially temperature-invariant reference voltage.
A bandgap voltage reference circuit is typically used to provide a voltage reference for other circuits to use in performing their intended operations. Generally, it is desired that the reference voltage generated by a bandgap circuit is substantially invariant. This is so even if there are substantial variations in the environment temperature. Thus, many, if not all, bandgap circuits incorporate temperature compensating circuitry in order to generate a substantially temperature-invariant reference voltage.
FIG. 1 illustrates a schematic diagram of a prior art bandgap voltage reference circuit 100. The bandgap circuit 100 consists of PMOS transistors Q11, Q12, and Q13, and NMOS transistors Q14 and Q15 configured as current mirrors to generate substantially equal currents I11, I12, and I13. The bandgap circuit 100 further consists of resistor R11 and diode D11 coupled in series with PMOS transistor Q11 and NMOS transistor Q14 to receive current I11, a diode D12 coupled in series with PMOS transistor Q12 and NMOS transistor Q15 to receive current I12, and resistor R12 and diode D13 coupled in series with PMOS transistor Q13 to receive current I13. The diodes D11, D12, and D13 are forward biased with their cathode coupled to ground terminal. The output reference voltage of the bandgap circuit 100 is generated at the node between the PMOS transistor Q13 and resistor R12.
The temperature compensation of the output reference voltage of the bandgap circuit 100 operates as follows. The current I12 generates a voltage V13 across the diode D12. The voltage V13 has a negative temperature coefficient xe2x88x92Txcex1V13. The current I11 generates a voltage V12 across the diode D11. The voltage V12 also has a negative temperature coefficient xe2x88x92Txcex1V12 that is more negative than the temperature coefficient xe2x88x92Txcex113 of voltage V13 (i.e. xe2x88x92Txcex1V12 less than xe2x88x92Txcex1V13). The current mirror causes the voltage V11 on the node between transistor Q14 and resistor R11 to be substantially equal to the voltage V13. Thus, the voltage VR11 across the resistor R11 (VR11=V11-V12) has a positive temperature coefficient +Txcex1R11 due to xe2x88x92Txcex1V12 being more negative than xe2x88x92Txcex1V13. Since the current I11 through resistor R11 is proportional to the voltage VR11 across the resistor R11, the current I11 likewise has a positive temperature coefficient +Txcex1I11.
The current mirror causes the current I13 to be substantially equal to the current I11. Therefore, the current I13 also has a positive temperature coefficient +Txcex1I13. It follows then that the voltage VR12 across resistor R12 has a positive temperature coefficient +Txcex1V12 since VR12 is proportional to the current I13. Additionally, the current I13 generates a voltage V14 across the diode D13 that has a negative temperature coefficient xe2x88x92Txcex1V14. The reference voltage VREF is the sum of voltages VR12 and V14, both of which have opposite polarity temperature coefficients. Thus, by proper design of the bandgap circuit 100, the reference voltage VREF can be made substantially temperature invariant across a particular temperature range.
FIG. 2 illustrates a schematic diagram of another prior art bandgap circuit 200. The bandgap circuit 200 operates similar to bandgap circuit 100. Briefly, the voltage V22 across the diode D22 has a negative temperature coefficient xe2x88x92Txcex1V22 and the voltage V21 across the diode D21 also has a negative temperature coefficient xe2x88x92Txcex1V21 that is more negative than xe2x88x92Txcex1V22. The operational amplifier U21 causes the voltage V23 at the positive terminal of the operational amplifier U21 to be substantially the same as voltage V22 across diode D22, which also has a similar negative temperature coefficient xe2x88x92Txcex1V23. Since xe2x88x92Txcex1V21 is more negative than xe2x88x92Txcex1V23, the voltage VR21 across resistor R21 has a positive temperature coefficient +Txcex1VR21, and accordingly the current I21 through resistor R21 also has a positive temperature coefficient +Txcex1I21. The current I21, as well as current I22 through resistor R22, are derived from the current I20 through PMOS transistor Q21. Thus, they all have a positive temperature coefficient. The reference voltage VREF is thus the addition of the voltage V22 and the voltage drop across resistor R22, both of which have opposite polarity temperature coefficients which can be made to cancel out.
A drawback of the prior art bandgap circuits 100 and 200 stems from the reference voltage VREF being a combination of two voltage drops in series. In bandgap circuit 100, the reference voltage VREF is a combination of V14 across the diode D13 and VR14 across the resistor R12. In bandgap circuit 200, the reference voltage VREF is a combination of V22 across the diode D22 and VR22 across the resistor R22. Because of this, the power supply voltage VDD needs enough headroom to accommodate both voltages that form the reference voltage VREF in addition to the source-drain voltages of transistor Q13 or Q21. The reference voltage VREF typically requires about 1.2V and the source-drain voltage of transistor Q13 or Q21 requires at least 0.2V. Thus, the minimum power supply voltage VDD required is about 1.4V, which makes the prior bandgap circuits 100 and 200 not compatible with emerging technologies that use VDD at significantly lower voltage than 1.4V, such as 1V.